Mutants of Streptococcus mutans which are defective in their ability to produce plaque in vitro will be studied. Glucosyl transferases of both parental and mutant strains will be compared by means of their isoelectric profiles utilizing both column and gel isoelectric focusing. The mutant strains also present altered colonial morphologies and by complementation analysis of the enzymes produced by various phenotypes it may be possible to correlate mutant enzymes with mutant cistrons. Labeled dextran molecules of low molecular weight will be used to determine whether the phenotypes are also related to cell surface specific glucan-binding receptor sites which are known to be present in wild type cells and apparently mediate aggregation.